We have a counselor using a public course, no student enrollments. She has posted announcements and they do not show in course navigation or on the homepage with the setting on to show Recent Announcements.
Are announcements not an option in a Public course?
Because there are no enrollments, there isn't a course card on the dashboard or students associated with the course, so no notifications are sent out either. Students will see the announcement (either on the course homepage if set or in the Announcements section) when they visit the course only.
We have counselors/campuses using institutional courses much the same way and it is a problem they would like solved. We have them submit important announcements be made at the campus level (sub-account) and that mostly works.
Additionally, we've suggested setting up grade level Reminds for more targeted communication.